Hydraulic valve-lash-adjusting elements serve to adjust the lash which forms due to wear or thermal expansion between the transmission elements of the cam lift and the gas-exchange valves of an internal combustion engine. This is intended to achieve a low-noise and low-wear valve train and the greatest possible conformity of cam rise curve and valve travel curve.
A hydraulic valve-lash-adjusting element for the valve train of an internal combustion engine is disclosed in EP 1 298 287 A2 and is characterized by the following features:                a housing has a blind bore in which a piston is guided with sealing clearance and is in pressure contact with a valve-actuating element and its cam;        the piston, together with the blind bore, defines a high-pressure space while a low-pressure space is located above the piston;        the pressure spaces are connected by at least one central axial bore in the piston, this axial bore being controlled by a control valve arranged in the piston.        
Control valves of this type are designed as non-return valves. They have a control valve ball which is acted upon by a control valve spring. In the case of the standard construction of the control valve, the control valve spring is acted upon in the closing direction. As a result, the control valve is predominantly closed and an idle travel of the valve-lash-adjusting element is omitted. There is even the risk of the same being pumped up and of a negative valve lash.
These disadvantages are avoided by control valves, the control valve spring of which acts upon the control valve ball in the opening direction. Hydraulic valve-lash-adjusting elements with a control valve of this type, on account of the reversed arrangement of the control valve spring, are called reverse spring hydraulic valve-lash-adjusting elements (RSHLA) or “normally open lash adjusters” (NOLA) because of the control valve which is open in the base circle phase. RSHLAs are distinguished by a positive effect on thermodynamics, pollutant emission and mechanical stressing of the internal combustion engine and are therefore increasingly used.
Since the RSHLA is closed by hydrodynamic and hydrostatic forces only by means of the lubricating oil flow which starts up at the beginning of the cam rise and flows from the high to the low-pressure space, the RSHLA always has an idle travel before the beginning of the valve travel. The magnitude of the said idle travel depends on the rotational speed of the engine and the length of the closing time of the RSHLA and the latter in turn depends on the viscosity or temperature of the lubricating oil. If a constant idle travel is desired, complicated measures are required at the control valve.
A further problem of the RSHLA is shown with reference to the design of the valve body spring of EP 1 298 287 A2, which forms the generic type. This valve body spring is designed in such a manner that the valve permits a fluid transfer between the high-pressure and low-pressure space during assembly of the RSHLA but closes as quickly as possible against the spring force of the valve body spring when pressure increases in the high-pressure space. This spring force must accordingly be relatively low. The spherical valve-closing body can therefore be set in rotation by a possible lateral incident flow and can be laterally displaced with the valve body spring. As a result, the closing force of the valve body spring and consequently the idle travel of the RSHLA are changed. In the extreme case, the valve body spring may pass into the seating gap of the control valve, which may lead to further variations in the idle travel and to the detuning or even to the total failure of the RSHLA and to the destruction of the valve body spring.